Continuous, reeled pipe, generally known within the energy industry as coiled tubing, has been used for many years. It is much faster to run into and out of a well bore than conventional, jointed, straight pipe.
Coiled tubing is run in and out of well bores using what are well known in the energy industry as coiled tubing injectors. The name derives from the fact that, in well bores, the tubing must be literally forced or "injected" into the well through a sliding seal to overcome the well pressure until the weight of the tubing exceeds the force produced by the pressure acting against the cross-sectional area of the tubing. However, once the weight of the tubing overcomes the pressure, it must be supported by the injector. The process is reversed as the tubing is removed from the well.
The only method by which a continuous length of tubing can be either forced against pressure into the well, or supported while hanging in the well bore or being lowered or raised is by continuously gripping a length of the tubing just before it enters the well bore. This is achieved by arranging two continuous chain loops on opposite sides of the tubing, in an opposing relationship. The continuous chains carry a series of gripper shoes which are pressed against opposite sides of the tubing and grip the tubing. Each chain is stretched between a drive sprocket and an idler sprocket. At least one of the two drive sprockets is driven by a motor to turn one of the continuous chains to supply injection or pulling force. The other drive sprocket may also be driven, typically by a second motor, to drive the second chain in order to provide extra power.
Coiled tubing has traditionally been used primarily for circulating fluids into the well and other work over operations, rather than drilling, because of its relatively small diameter and because it was not strong enough, especially for deep drilling. In recent years, however coiled tubing has been increasingly used to drill well bores. For drilling, a turbine motor is suspended at the end of the tubing and is driven by mud or drilling fluid pumped down the tubing. Coiled tubing has also been used as permanent tubing in production wells. These new uses of coiled tubing have been made possible by larger, stronger coiled tubing.
To handle the longer and heavier tubing, used in drilling, an injector must be capable of carrying much greater loads. Drilling sometimes progresses very slowly. Therefore, in addition to running the pipe into and out of the hole rates measured in tens or hundreds of feet per minute, the injector must also be capable of advancing the pipe at rates measured in inches per hour. Because of the required control, power for driving an injector used for drilling is usually provided by a high speed, low torque, hydraulic motor, rather than a low speed, high torque hydraulic motor. Low speed, high torque motors are conventionally used on injectors. A high speed, low torque motor must be coupled to the injector through a transmission with reduction gearing.
As shown, for example, in FIG. 1, a prior art coiled tubing injector 101 (shown in section) has a conventional configuration of two continuous chains 103 and 105, each looped around one of two upper sprockets 107 and 109, respectively, and on one of two lower sprockets (not visible). Sprockets 107 and 109 are mounted on shafts 111 and 113, respectively, and are keyed to the sprockets through splines 114. Each shaft is journalled on frame 115. Each chain carries a plurality of grippers 117. As the loops of the chains turn, grippers 117 are pressed against opposite sides of continuous tubing 119. A high speed, low torque hydraulic motor 121 is coupled to shaft 111 to supply power to turn chain 103 through a brake 123 and planetary gear box 125. Similarly, high speed, low torque hydraulic motor 127 is coupled to shaft 113 through brake 129 and planetary gear box 131 to drive chain 105. The axis of each motor and planetary gear box is aligned with the axis of the shaft they turn. Because of the diameter of the planetary gear boxes and motors, a motor and planetary gear box must be mounted on opposite sides of the chains in order to synchronize movement of the respective chains, intermeshing timing gears 135 and 137 are mounted on shafts 111 and 113. The timing gears are capable of transmitting only as much power as is required to maintain timing between the chains. As is evident from the illustration, as the sizes of the motors and gearing increase to handle larger loads, so too will the depth of the coiled tubing injector, as measured perpendicularly with respect to the plane of rotation 139 of the sprockets and chains, in directions indicated by arrows 141a and 141b.